One alternative would be a manually set timer that would be initiated by a person who was able to evaluate the amount of ice, and estimate the time to get rid of it. The problem of course is that it would require human judgement, skill, and accountability, the three things that ISO 9000 seeks to remove from the business environment completely.

A photo sensor does seem like a good idea, although I would want a backup set fpr a less frequent interval for when the sensor system fails periodicaly, which it will, of course. Whart would be quite interesting now is a detailed description of how the frost detector works. Not just a general description, but one that would provide an understanding. Of course, that assumes that the design is adequately protected.

The timer based comment is correct in that there is a minimum time between defrost operations. Home refrigerators with electronic controls base their defrost times on door openings, temperature, humidity and previous defrost cycle time. The calculations that detwermine time between defrosts is based on years of research to discover the maximum time a refrigerator can run efficiently between defrosts coupled with the previously noted information.

The defrost circuit also has a thermal sensor included in the defrost circuit to determine when the coil temperature is above freezing to terminate the defrost operation. I believe this was mentioned in a previous post.

There are other issues involved in defrosting coils such as how long to allow the evaporator to drip to remove water, maximum time to allow a defrost to keep from warming the freezer area, etc.

For years a thermostate has been used to control the icing situation. After a coling cycle the fan is kept on until the thermostate indicate that the temperature is above 32°F, an indication that the coils are ice free. Simple, reliable and not that expencive.

It's not necessarily advanced, more like clever. As far as I know, there's no sensor now in use to detect ice on cooling coils in commercial refrigerators, so something that can do the job and save energy looks like an advancement.

Sure, someone could do something similar with a few bits and pieces and a PIC MCU, but they didn't, right?

Health codes require regular cleaning of commercial refrigerators used for food, so cleaning the sensor would become part of the routine. Also, I bet the sensor calibrates itself so if any dust settles on the coil--unlikely in a closed environment--the sensor calibrates for the small change.

What happens over time? How does the system know the sensor beam path is the same over time?

One sometimes sees air jets used to keep the lenses of optical sensors clear in dusty/dirty environments. What happens in this environment? Is the sensor itself heated to prevent ice or condensation inside the sensor, and to keep the lens clear?

This type of sensor would make sense in commercial refrigeration units rather than in home refrigerators or freezers. The cost would make it too expensive for standard fridges. As a youngster I remember my mother defrosting our refrigerator. The built-up ice would get so thick she would have to put pans of hot water in the open freezer compartment to melt it. Meanwhile the frozen food sat wrapped with ice cubes in newspaper in a dish drainer. It was quite a process. I'm glad we no longer have to do that!

Anyone who has experienced "frozen" coils in an air conditioner understands the need for s frost sensor. The AC continues to run because the room thermostat indicates a high temp. All because insufficient air gets through--or around--the coils. I grew up on the east coast where humid summer air caused a lot of frozen freezer-case problems. --Jon

Jon, you are definitely right that this is one of those "why didn't I think of that?" inventions. It's hard to believe that, up to now, nobody had come up with anything more than a timer for this application.

Thanks for the introduction to rime ice, Jon. I definitely have seen traces of it in my refrigerators and freezers over the years and now I know what it is!

Sounds like a practical use of sensor technology, which while helping with energy savings might also help reduce the number of service calls related to finding that frozen substance taking over the inside of your fridge. On the other hand, given what we see regularly in The Made for Monkeys column about new appliance train wrecks, I wonder if this is just another high-tech gizmo that complicates matters and sets the appliance up for a less than desired failure rate.

Switched-capacitor filters have a few disadvantages. They exhibit greater sensitivity to noise than their op-amp-based filter siblings, and they have low-amplitude clock-signal artifacts -- clock feedthrough -- on their outputs.

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